Polypropylene is widely used for various purposes in the form of film, fiber, and molded articles because of its low cost of production, high mechanical strength, high heat resistance, high oil resistance, and excellent electrical characteristics.
However, polypropylene is inferior to other vinyl polymers, especially polystyrene, in rigidity, processability in sheeting, and surface characteristics. In the field of packaging vessels, for example, polypropylene is less rigid and less processable in sheeting than polystyrene but, on the other hand, superior to polystyrene in heat resistance, oil resistance, and moisture resistance. Accordingly, a molding material having the merits of polypropylene combined with those of polystyrene in good balance would find new uses in the fields where each resin has been inapplicable.
Blending polypropylene with polystyrene or polymerization of styrene in the presence of a propylene polymer has been attempted. For example, U.S. Pat. No. 4,097,554 discloses modified polypropylene obtained by polymerizing a styrene monomer in an aqueous suspension in the presence of a propylene polymer. U.S. Pat. No. 5,140,074 discloses modified polypropylene obtained by activating a particulate propylene polymer with a peroxide and polymerizing a styrene monomer in the presence of the activated propylene polymer in a solid phase system under such conditions that the propylene polymer may retain the particle form.
However, polypropylene and polystyrene are incompatible with each other. When these resins are merely blended, the polystyrene shows insufficient dispersibility and undergoes phase separation due to poor interfacial adhesion between the two resins, only to provide molded articles which are unsatisfactory in transparency, rigidity, impact resistance and the like and are therefore unsuited to practical use. When a styrene monomer is polymerized in an aqueous suspension system or a solid phase system in the presence of a particulate propylene polymer, the production steps required for, for example, prevention of blocking, control of polymerization temperature, and drying of a product are complicated, which has made it infeasible to industrialize the product as a general-purpose polymer.
Thus, conventional techniques for introducing a styrene polymer into a propylene polymer are unsatisfactory from the standpoint of the polymer property and the process thereof. The drawback of the conventional techniques is ascribed to insufficient dispersion of polystyrene in a propylene polymer. While a styrene-grafted propylene polymer is desirable for obtaining a sufficient disperse system of polystyrene in propylene polymer, it is difficult to form a graft polymer by the above-mentioned processes probably because of the high crystallinity of polypropylene, which means primarily a high melting point and secondarily insolubility in a styrene monomer.
In the study on modified propylene polymers with improved transparency, rigidity and moldability, the present inventors previously found that an aromatic vinyl monomer-grafted polymer excellent in various properties can be obtained by a process basically comprising polymerization of an aromatic vinyl monomer in the presence of a molten propylene polymer in, e.g., an extruder and that the process is easy to carry out on an industrial scale (JP-A-4-359910, JP-A-5-155947, and JP-A-5-239158, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, as a result of further study, the above process turned out to involve the following disadvantage. That is, when a propylene polymer is fed to an extruder and the like and melted, while a vinyl monomer is fed to the middle of the extruder by means of a pump and melt-kneaded with the molten propylene polymer and polymerized, infiltration of the vinyl monomer into the propylene polymer at ambient temperature is insufficient. As a result, the propylene polymer cannot be smoothly fed to the extruder, or the resulting modified propylene polymer shows insufficient compatibility.
When a propylene polymer and a vinyl monomer are premixed and then melt-kneaded and polymerized in an extruder, it takes time for the vinyl monomer to be infiltrated into the propylene polymer at ambient temperature. If the propylene polymer is used in a form having a small specific surface area, such as a form of pellets, the vinyl monomer is not sufficiently adsorbed onto the propylene polymer, tending to separate and run off the blend, or the premix fed to an extruder tends to slide on the screw and fails to bite, namely, poor reactive processability during extrusion (hereinafter referred to as reactive extrusion processability). It follows that the vinyl monomer cannot achieve a high polymerization rate and a high graft ratio, and that the resulting modified propylene polymer cannot have a high proportion of a component derived from the vinyl monomer and is inferior in uniformity and transparency.